The present invention relates generally to measurement test instruments for characterizing a transmission medium and more particularly to an improved time domain reflectometer and method for reducing line charge in the transmission medium during characterization.
A time domain reflectometer (TDR) launches interrogating energy pulses into a transmission medium, such as shielded and unshielded twisted pairs, coaxial cables, and the like, at a given pulse repetition rate, depending upon the designated range of the TDR. During the periods between pulses, acquisition circuitry samples the cable to acquire data representative of reflections from flaws, discontinuities, or breaks in the cable. The reflections in the cable are timed from the time of transmission of the energy pulse to determine the range from the transmitter to such flaws, discontinuities, or breaks. Reflections may represent changes in wire gauge, splices, moisture in the cable, and the like. The acquired data is processed and displayed as a waveform trace on a display device, such as a cathode-ray-tube, a liquid crystal display, or the like.
A TDR notes any changes in the characteristic impedance of the cable under test. For a telecommunications copper facility or plant, the characteristic impedance is typically between 100 and 125 ohms. Most unshielded cables fall between 100 and 105 ohms. Shielded cable like T1 is typically about 125 ohms. Any change in the cable's impedance is displayed on the TDR display device as an upward bump, downward dip, or some combination of both deviating from a horizontal trace.
The TS100 and the TV110 Time Domain Reflectometers, manufactured and sold by Tektronix, Inc., Wilsonville, Oreg. and assignee of the instant invention, are examples of TDRs for respectively characterizing telephone twisted pair cables and coaxial cable, such as in CATV and the like. Both instruments generate 1/2 Sine interrogating energy pulses, such as shown in FIG. 1. The amplitude and pulsewidth of the applied interrogating pulses are varied as a function of a number of factors, such as cable type, cable length, and the like. The TS100 generates interrogating pulses having a maximum amplitude of up to 20 volts and pulsewidths from 5 nsec to 300 nsec. The TV110 generates interrogating pulses having a maximum amplitude of 15 volts and pulses widths of 6 nsec and 12 nsec. To extend the range of the TDR requires increasing the energy of the applied pulse in order to increase the amount of energy reflected from an event. This can be done by increasing the amplitude of the launched pulse, increasing the pulsewidth, or a combination of both. Increasing the energy launched into the cable under test has the undesirable effect of placing a charge on the transmission line being tested which may mask or obscure the reflective information close to the instrument. FIG. 2 shows overlain waveform traces, A and B, of return signals acquired using the 1/2 Sine waveform of FIG. 1 launched into 15,000 feet of 24 gauge twisted pair transmission line. The interrogating pulse produces a high amplitude reflective pulse 10 at the instrument/cable under test interface. Due to line charging, the interface reflective pulse decays exponentially as shown by the region 12. Events within the line charge region, such as the short in waveform trace A at 14 and the open in waveform trace B at 16, may be masked or obscured by the line charging. In the waveform traces of FIG. 2, the scale has been increased to show the existence of the shorted and open events. In general use, the TDR vertical scale is adjusted to show the complete waveform events without clipping the tops or bottoms of events. This would generally obscure events within the line charge region.
What is needed is a measurement test instrument, such as a time domain reflectometer, and method for reducing the line charge placed on a transmission medium, such as a twisted pair lines and coaxial cables, by interrogating energy pulses. The measurement test instrument should provide increased viewability and interpretation of waveform data representing the return reflections in the transmission cable, particularly when the return reflections are small in amplitude and instrument amplifier gain is large to adequately view the reflections.